This invention relates to a moisture barrier assembly for the microphone unit of a telephone handset, particularly that of a payphone.
During wet weather, the handset of a payphone may be subject to partial or total loss of forward transmission resulting from the freezing of moisture (probably from a user's breath) which has entered into the transmitter.
In order to alleviate this problem, it is known to fit a moisture barrier into the transmission housing of a payphone handset. A known moisture barrier is a simple disc made of a thin plastics film (for example melinex). Typically, such a disc is held between the parts of a two-piece crimped transmission housing, with the disc overlying the microphone. Unfortunately, the disc tends to separate from the housing, probably due to thermal shock and/or an inaccurate disc fitting process. This disc seperation permits moisture to creep round the disc edge and into the transmitter. Consequently, transmission loss occurs in freezing conditions. The sound holes in the mouthpiece of the handset may also be blocked, which increases the transmission loss.
In an attempt to overcome this problem of moisture creep round the disc edge, another form of known moisture barrier has a thin film plastics disc provided with a surrounding annular support ring. The support ring is self-adhesive so that it can seal against the front of the transmission housing of a payphone handset. The main disadvantage of this type of moisture barrier is that the disc is too close to the mouthpiece, so that any moisture collecting between the disc and the mouthpiece tends to adhere the disc to the mouthpiece, and this leads to loss of transmission. There is also the possibility of the disc being sucked back onto the microphone, which would also lead to transmission losses.
The aim of the invention is to provide an improved moisture barrier assembly for a payphone handset.